1. Field of the Invention
The present invention relates to an optical pickup device which writes information data on an optical disc and reads written information data from an optical disc.
2. Description of the Related Background Art
In an optical disc which is used as an optical recording medium, a transparent layer having a predetermined thickness is formed on a recording surface of the optical disc so as to cover and protect the recording surface. In an optical information recording and reproducing apparatus, by emitting a reading beam light or a recording beam light to the recording surface through the transparent layer of the optical disc, the reading and recording of information data for the optical disc is performed.
However, in manufacturing many optical discs, it is difficult to fix the thickness of the transparent layers to a prescribed value and hence, a thickness error of several tens of xcexcm is usually generated for each of the discs. Further, as for a DVD (Digital Versatile Disc) which has a plurality of recording layers in a single disc, for example, there is a transparent layer between a surface of the disc, to which a laser beam is applied, and a recording layer, and there is another transparent layer between the one recording layer and another recording layer. Thus, the distances from the surface to the respective recording layers differ from each other (see Japanese Laid-open Patent Publication 16214/1999).
Accordingly, there is a problem that since the difference in thickness of the transparent layer portion for the respective recording layers exists, a spherical aberration is not generated at a constant level as shown in FIG. 1 and hence, the information recording accuracy and the information reading accuracy are deteriorated.
It is therefore an object of the present invention to provide an optical pickup device which can overcome the problem and can accurately perform writing and reading of information to/from an optical disc by compensating for the spherical aberration.
The optical pickup device according to the present invention comprises an optical system which includes a first optical path which introduces a laser beam emitted from a light source to a recording surface of an optical disc through an objective lens and a second optical path which introduces the laser beam reflected on the recording surface of the optical disc to an optical detector through the objective lens, and an optical element which is disposed at least in the first optical path such that the optical element is movable in an optical axis direction of the laser beam.